Audio frequency thermionic amplifier



Sept. 19, 1933. w s $M|TH ET AL 1,927,377

AUDIO FREQUENCY THERMIONIC AMPLIFIER Filed Sept. 22, 1928 3 Sheets-Sheet 1 w. s. SMITH N. w. McLACHLAN,

Mr Atto neys,

Sept. 19, 1933. w 3 SMITH r AL 1,927,377

AUDIO FREQUENCY THERMIONIC AMPLIFIER Filed Sept. 22, 1928 3 Sheets-Sheet 2 INVENTORS 3- SMITH W. MCLACHLAN than Attorneys Sept. 19, 1933.

W. 5. SMITH ET AL AUDIO FREQUENCY THERMIONIQC AMPLIFIER Filed Sept. 22, 1928 3 Sheets-Sheet 3 Patented Sept. 19, 1933 PATENT; OFFICE auirro FREQUENCY 'rnnminomc AMPLI- FIER Willoughby Statham Smith, Bencllams, Newton Poppleford, and Norman William McLachlan,

' London, England Application September 22, 1928, Serial I IND-'1 307,618, and in Great Britain October 25,1927

4 Claims. (01. 179-171) This invention relates tn audio' frequency thermionic amplifiers for use in submarine and. the like'cable systems.

The invention has for its object to provide anamplifier which shall enable "reasonably undistorted telephoniccommunication to be efiected over long lines of' cable. With apparatus as at present known, wired telephonic communication can only be effected over'long distance by inserting amplifiers, generally termed repeaters; at frequent intervals inthe line, in order to overcome the very considerable frequency attenuation and weakening which a long length of cable effects. .Indeed, in common telephone practice, repeaters are inserted at intervals equivalent to not more than 20 miles of standard cable, in the case of so-called two-wire repeaters, or at intervals equivalent to not more than 40 miles of standard cable, in the case of four-wire re peaters.

Another object of our invention is to provide a circuit arrangement for coupling the output circuit of one screen grid valve with the input circuit of a succeeding screengrid valve comprising a resonant coupling device whose im pedance is low as compared with the internal impedance of the valve preceding it and which is tuned to afrequency at or near the upper limit of the range of frequencies .to which the valve circuits are responsive.

Other and further objectspf our invention reside in the provision of a circuit arrangement for an amplification system as set forth in the following specification and shown in' the accompanying drawings, in which:

Figure 1 diagrammaticallyshows an amplifier circuit embodying the principles of our invention; Fig. 2 illustrates a modified form of circuit for one of the stages of amplification in the amplifier system of our invention; Fig. 3 shows one form of coupling circuit in which a resistance and reactance are disposed in series and connected in parallel with the output circuit of one electron tube and the input circuit of a succeed- ,jing electron tube in, accordance with the principles of our invention; Fig. 4 shows'a further modified form of amplification circuit. embodying our invention; Fig. 5 illustrates aset of curves. employed for explaining the operation of the j lcircuits of our invention; Fig. 6 shows one form of filter employed in the cathode circuit of the amplifier system of our invention; Fig. '7 shows a form of filter employed in the anode circuit of the amplifier system of our invention; Fig.

8 shows the application of our'invention to a balanced push-pull amplifier circuit;Fig. 9 shows one form of core lamination employed in the coupling transformer utilized in the amplification system of our invention; Fig. l0shows a complete amplification system embodying the circuits of our invention; Fig. 11 showsa modified form of coupling circuit which may be employed in the amplification system of our invention; and Fig. 12 illustrates schematically one form of shielding meanswhich may be "employed'b'etween the system of our invenstages of the amplification tion. 7 7

It will readily be appreciated that in many systems, more especially in the case of submarine. cables, it is impossible'or impracticable to insert repeaters at such frequent intervals, and this fact has prevented the utilization graph cables for telephony.

' The present invention is directed to the employment of the so-called screened grid thera Inionic valve in atelephoneamplified. The socalled screened grid thermionic valve comprises a cathode, a control grid, an anode and a screening grid located between the control grid and the anode, and serving to act as an electrostatic;

screen therebetween, whereby the interelectrode capacity normally present in a triode between the. input or control grid electrode and the output or anode electrode, is substantially reduced.

According to this invention a thermionic airiplifier, suitable for use at speech'frequencies, in conjunction with glonglengths of submarinecable or other line, comprises in'combination one or more screened grid thermionic valves and one or more resonant transformers or equivalent in ductive devices.

The arrangementshould be such that the amplification-frequency characteristic of the amplifier as awhole is a rising curve substantially opposite to the falling frequencyattenuation curve vof the cable with which the said amplifier is associated. The effective resonant frequency ofthe transformers or equivalent devices (i. e. the resonant frequency whenthe said transformers are conof many telenected in circuit) should lie between the values of 1,200 and 7,000 cycles per second, and where there is a plurality of stages in the amplifieras will normally be the case-the resonant frequencies of the transformers or equivalent devices in the various stages may conveniently be of difierent values.

Screened grid valves employed in carrying out the present invention may, if desired, be of the kind comprising (in addition to the cathode, control grid, anode and screening grid) an auxiliary screening grid interposed between the main screening grid and the anode and maintained in general at or about cathode potential. Such screened valves, which are at the present time generally termed pentodes, offer the advantages of high internal resistance and high mutual conductance, while, owing to the presence of the auxiliary grid (which minimizes or eliminates secondary emission effects and the consequent negative resistance portion'of the anode-voltage anode-current characteristic, commonly obtained with screened grid valves other than pentodes), a high magnification is obtainable and large grid swings may be permitted Without substantial distortion. V I

Amplifiers in accordance with this invention may be incorporated at the receiving end of a submarine cable or the like and/or they may be employed in conjunction with a transmitter feeding audio frequency current into said cable or the like, whereby the whole or part of the signal shaping to compensate for the effects of attenuation in the said cable is effected at the input thereto. Obviously, where amplifiers in accordance with this invention are employed both at the input and output ends thereof, they should be regarded, from the point of viewof attenuation correction, as a single amplifier having a part at each end of the cable and serving between them to provide the required correction.

Amplifiers in accordance with this invention may comprise one or more push-pull stages of amplification, and it is found that where such a stage includes a single cored push-pull transformer, the resonance frequency of one half of the transformer secondary may appreciably modify that of the other, due to the coupling therebetween. For example, in a case in which a transformer secondary comprised two-coils, it was found that the addition of the second coil reduced the resonance frequency of the first coil, which had been 5,000 cycles per second, to 3,100 cycles per second. In some cases it maybe desirable to avoid this and to this end two electrostatically and magnetically uncoupled transformers may be employed in place of a push-pull transformer as the coupling device of a push-pull amplifying stage.

In one such construction, the secondary of one transformer is connected between the grid and cathode of one of a pair of push-pull connected valves, the secondary of the other being connected between grid and cathode of the other valve, and the arrangement being such that substantially the only coupling between the transformer secondaries is at the direct connection to the common cathode lead. The valves in the push-pull amplifier should have such values of bias potentials applied to their grids as to bias them to work on either side of substantially the mid point of the straight portion of their characteristics.

It will be appreciated that considerable advantage may be derived in practice by making an amplifier in accordance with this invention adjustable as to frequency-amplification characteristics, so as to be readily matched to suit different cables having different frequency attenuation characteristics. More particularly is advantage to be obtained in the case of receiving amplifiers. To this end one or more variable resistances may be connected across the primaries and/or secondaries of one or more of the coupling transformers (or in any other electrically equivalent position) in an amplifier of thekind referred to, in cases in which it is desired to vary the shape of the characteristic, while one or more condensers may be connected in similar positions in cases in which it is desired to alter the effective resonant frequency of an amplification stage. The resonance may also be adjusted by employing an adjustable open iron core.

Preferably both adjusting condensers and resistances are provided, and a variable resistance of high value is also preferably provided in series with the primaries of one or more of the coupling transformers, in order to effect control of actual magnification. If desired, such condensers and/ or resistances may be associated with tertiary windings, provided on the transformers, instead of with the primaries or secondaries thereof.

For the purpose of effecting volume control or for correction purposes, there may be included in the anode circuit of each of one or more of the thermionic valves in an amplifier in accordance with the invention, a resistance and a reactance (which may be short-circuited), the connection to the grid of a'succeeding valve being tapped from the said resistance. Alternatively, volume control may be effected by means of a variable tapping upon a grid leak.

The valve in whose anode circuit the resistance and reactance are included, and which may be either a so-called. screened grid valve or an ordinary triode, should be of fairly high alternating current resistance.

In one arrangement of the kind, the anode of a triode is connected through a resistance and thence through an iron-cored choke of about 25 40 henries in parallel with a condenser, to the anode battery. The condenser should be such as to bring the natural frequency to the anode ciredit from 3000. to 7000 cycles per second. A variable tapping is provided upon the resistance and is connected through a blocking condenser to the grid of a succeeding valve.

. In practice, the resistance and tapping may be employed as a volume control, the choke being short-circuited or the choke, capacity and resistance together may be employed for correction purposes.

If desired there may be provided in one or more of the stages of an amplifier in accordance with the invention a variable resistance, the said resistance being inserted between the anode of an amplifying screened grid valve and the primary of the resonant transformer or equivalent inductive device therewith associated, and serving to adjust the damping in cases where the valve has not the best value of impedance. Obviously also the damping may be reduced by increasing the resistance. i

If desired, also, resistances may be connected in series in the grid circuit or circuits of one or more of the amplifying valves and/or capacities connected in shunt across one or more of the said grid circuits. Such resistances and capacities re duce or eliminate tendencies to self-oscillation in a multi-stage amplifier. Care must be exercised, however, that these resistances do not appreciably attenuate the upper speech frequencies. Again, in some cases advantage will be obtained by providing external neutralizing circuits for balancing the effective self-capacities (small as these are) of one or more of the screened grid valves. One such arrangement consists in providing a tertiary winding (which should have as low a mutual capacity as possible to the other windings) upon a resonant transformer, the said tertiary winding being connected at one end through a small variable balancing condenser to the control grid of the associated valve, and at the other to the cathode thereof or vice versa. In cases in which resonant chokes are'employed to replace resonant transformers, additional windings coupled to such resonant chokes may be utlized for neutralizing purposes in place of the tertiary windings referred to above. Obviously other neutralizing arrangements may be employed; for example, those described in our British Patent No. 290,35Laccepted May 10, 1928.

- If'desired, in constructions embodying neutralizing arrangements for screened grid valves, in place of a single choke and associated condenser, a plurality of chokes and associated condensers may be provided in series, each choke and associated condenser having preferably a different resonant frequency. Where such a plurality. of chokes is employed, in each of a plurality of amplifyingstages, a neutralizing choke is preferably associated with only one choke in each stage, the said last mentioned chokes being of different frequency in each stage. In some cases, however, for example, in arrangements such as those described in our British Patent No. 290,35l, the Whole choke system associated with each valve should be neutralized. In this manner any tendency to feed back will occur at a different fre- .quency in each stage. Similarly, where resonant transformers are employed, more than one may be provided in each stage, but in such case only one in each stage is associated with a neutralizing circuit, except, of course, where the method described in our British Patent No. 290,351 is employed, the one so associated being different, i. e.

non-corresponding in each stage.

In another modification, an amplifier is split at one or more of its amplifying stages into a plurality of amplifying branches, each branch This has the result of reducing amplification at' low frequencies but not at high frequencies. Suitable values of chokeand condenser have been found to be respectively 1000 henries and .0002 microfarad. The voltage across the choke, of course, is not passed on to the next amplifying stage.

The present invention enables very large amplification to be effected per amplifying stage (a value exceeding 400 has been obtained, but in practice somewhat smaller values are employed),

and thereby enables speech to be transmitted and received satisfactorily and by means of comparatively small amplifiers over cables or other lines .of such length that clear speech thereover would be impossible with known apparatus.

Referring to Fig.1 of the drawings, which shows in diagrammatic form a'receiving fourstage amplifier in accordancewith the invention, the input from the cable (not shown) is passed through an input transformer 1 (which may, for example, have a step-up ratio of 20 or 30 to 1, and may have a rising characteristic) whose secondary is effectively connected between the control grid 2 and cathode 3 of a screened grid thermionic valve 4, whose screening grid and anode are indicated at 5 and 6 respectively. Connected in the anode circuit of the valve 4 is a special transformer '7, so designed that the overall voltage-frequency characteristic of the said transformer '1 and valve 4 has a resonance peak in the neighbourhood of 3500 cycles per second, the said characteristic rising to this point. Owing to the high internal resistance of the screened grid valve, the damping is quite small and a comparatively sharp resonance curve is obtained, while the very low effective self-capacity of the said valve permits of large magnification being obtained without self-oscillation.

fhe second stage of the amplifier, may, if desired, be similar to the first, but in the arrangement shown in Figure 1, it is constituted by a. triode 8 which is resistance capacity coupled to the thermionic valve in the third stage by means of resistances 9, 10 and a coupling condenser-. 11 (of about .0003 microfarad capacity) which is preferably adjustable so that a desired degree of frequency attenuation may be readily obtained.

If desired, resistances 12, 12, 12", 12", may be inserted in order to prevent spurious oscillations, due, for example, to inductance and capacity in the wiring.

If desired, one or more of the resistances l2, 1

12', 12", 12", may be made variable (for example, the resistance 12, as shown), and thus serve also to adjust the damping. The condenser 27, in conjunction with the resistance 12 con--r Y nected to the grid of the valve 8, serves to reduce or eliminate any tendency to self-oscillation, while the. resistance-23 and capacity 14 serve respectively to vary the shape of the characteristicand to alter the resonant frequency ofthe transformer coupling the last two valves of the amplifier, as may be desired. 29 Ba variable resistance of large; value and serves to effect magnification control.

Since the amplifier is represented in conventional diagrammatic form, the remainder of the circuit will be understood from the said figure without further explanation.

In the modification indicated in Figure 2, which shows 'a first stage only, the transformer 7 is replaced by a choke 13, andvariable smaller condenser 11. The circuit is, in other respects, like that of Figure 1.

In another modification, shown in Figure 3, which also shows two stages of an amplifier, a resistance 30 and a reactance constituted by an iron-cored choke 31 and shunt condenser 32, the said choke being adapted to be short circuited, if required, by a key 33, are included inthe anode circuit of an amplifying valve and the connection to the grid of the succeeding valve which is here shown as a pentode, is tapped upon the said resistance. The natural frequency of the anode circuit comprising the choke 31, which may be of an inductance ofabout 25 to 40 henries, should be'approxiniately 3000 to 7000 cycles per second. The resistance and tapping may be employed as a volume control with the choke short-circuited, or with the short-circuit removed the choke, capacity and resistance to- 5 gether may serve for correction purposes.

In the further modification illustrated in Figure 4, the amplifier comprises four generally similar stages, each constituted by a screened grid valve 4, 4, 4", or 4", the said stages being coupled by transformers 7, 7 and 7". Like parts in the successive stages are indicated by like reference numerals to which are affixed a number of dashes appropriate to the stage number.

The last two stages differ from the preceding stages in that the last stage comprises a pentode,

While the transformer 7" is provided with a tertiary winding 34 (which should have as low a mutual capacity as possible to the other windings) which is connected through a balancing condenser across grid and cathode of the penultimate valve, whereby the self-capacity of said valve may be neutralized.

The transformers 7, '7 and '7" are preferably designed so that their effective resonant frequencies (when in use in the amplifier) are different;

for example, the transformer 7 may advantageously be made to exhibit the characteristic curve A of Figure 5, the transformer '7', the curve B of N Figure 5, and the transformer '7", the curve C of Figure 5. In these curves, which do not represent the best results which have been obtained, the ordinates are voltage amplifications measured as the ratios of output potentials of a transformer to input potentials to a valve associated therewith, and the abscissae are frequencies in cycles per second. If desired, variable condensers such as that shown at 14, may be connected across the secondary of one or more of the transformers '7,

7, 7", so as to adjust the resonant frequency.

" Further, the presence of such condensers reduces noise. Curve A, for example, was obtained with a value of .0002 microfarad across the secondary of transformer '7. The internal resistances of q the valves employed in the tests to obtain these curves were between 150,000 and 400,000'ohms for a grid bias potential of -1.5 volts, screening grid potentials being in the neighbourhood of +80 volts and anode potentials about 120 volts.

To ensure stability in the amplifier, battery coupling, i. e. coupling due to the internal resistance of the batteries employed to supply the valves with energy, should be avoided as far as possible. An arrangement in which battery coupling through a common cathode battery is avoided is shown in Figure 6, in which 15 is the cathode battery, and 16 and 17 chokes and condensers respectively constituting an audio frequency filter.

Figure 7 shows a high tension battery arrange- .ment, whereby a multi-stage amplifier in achigh tension current from a single battery with a minimized risk of battery coupling through the said common anode battery. Referring to this figure, 18 and 19 are chokes and condensers respectively, constituting audio frequency filters, 20 is the common anode battery, 21 the common negative or cathode lead therefrom, 22 the lead for providing screening grid potential (say volts), 23, 24 and 25 the first, second and third anode feeds (say, 120 volts each) and 26 the fourth or final anode lead (say 200 volts).

If desired, filters may be inserted in the leads to all the screening grids and to all the anodes and cordance with the invention may be supplied with also in the leads supplying control grid bias and cathode heating current.

Referring now to Figure 8, which shows in conventional circuit diagrammatic formone form of transmitting amplifier in accordance with the invention, signals from a land line or other source are applied via a transformer 1 to the grids and filaments of a pair of so-called screened grid valves 4a, 4b, arranged back to back. In the lead connecting the anodes of the valves 4a, 4b, is situated the primary of a transformer '7 having a rising characteristic and designed in accordance with the invention. The secondary of the transformer is formed in two halves, each half being connected as shown across the input of one of a pair of valves4a, 4b, arranged in push-pull fashion. In the lead connecting the anodes 6a, 6b, of the Valves 4a, 4b, are situated a pair of variable resistances 36a, 360, connected each in series with identically tuned chokes 37a, 37b, whose resonance frequency in conjunction with any added capacity 38a, 38b, is designed to be between 1200 and 7000 cycles per second. If desired, the effective inductance and. capacity of the chokes may be variable as shown.

It will be seen that variation of the values of the chokes and resistances will serve to vary the frequency correction and amplification characteristics. If desired, the chokes may be arranged to be short-circuited and the resistances used 105 solely for amplification and volumev control. Again, if desired, the pair of resistances and the pair of chokes may be adjusted, by a single-handle for each pair.

The valves 4a, 4b, are coupled to succeeding 110 valves 4a, 4b, as shown, by condensers 39a, 39b, and resistances 40a, 40b, and the said resistancesand condensers may, if desired, also be used to assist in shaping. The anodes of the valves 4a", 4b" are connected through the primary of a transformer 7a whose secondary feeds, as shown, into the cable 41, and whose windings are screened by a screen 42. The windings of transformer 7 are similarly screened at 42. The middle point of the secondary winding (or of a resistance 43 connected thereto) is earthed.

The ratio of the transformer 7a is preferably according to the expression where the effective internal resistance of the valves 9, 9, k: the transformer ratio, and Z:

the characteristic impedance of the line 41 at the 130.

design frequency.

If desired, the primary inductance of the transformer 12 may be reduced to attenuate the lower frequencies and accentuate the upper frequencies. This result may also be achieved by making transformer 12 of a reduced ratio, i. e. one less than thin metal foil 7f, to act as a short-circuited winding, the metal foils being connected together and to earth as shown at 7g. 2

The chokes C and condensers K shown in the figure, serve to prevent alternating current passing through the anode and screen batteries, and thereby coupling the valves together, due to battery impedance. I

It will be seen that if it be assumed that all speech frequencies are equally representedfi. e. in their correct proportions) at the input to the valves 4a, 4b, then the input to the cable 41 will contain a much greater proportion of high than of low frequencies, and this will obviously cornpensate to a greater or lesser degree for the greater attenuation of the cableatthe higher frequencies. Since the attenuation is greatest at the higher audio frequencies, the length of the initial or head end part of the cable, which ,must withstand heavy currents, is much less with a shaped input (i. e. an input obtained from an amplifier such as that described above) than it would be if all frequencies were equally represented, for inthelatter case large low frequency currents would persist to a considerable distance, necessitating a greater head and length.

The number of valve stages and degree of correction required depends upon such design considerations as the characteristics and length of cable, upon the current which can safely be supplied-to the cable without altering the properties of the loading material, and upon the characteristic of the receiving amplifier. In other words, the transmitter and receiver are designed to fit the cable, so that-the output at the last valve ofthe receiver shall have a' certain frequency characteristic for good reproduction.

In some cases it may be found preferable to substitute for the single push-pull transformer '7 of. Figure 8 a pair of magnetically and electrostatically uncoupled transformers of which the secondary of one is connected between grid and cathode of the valve 4a, the secondary of the other being connected between grid and cathode of the valve 41). should be such that the only coupling between the transformer secondaries is at the direct connection to the cathodes of the valves 4a', 4b.

In all constructions of amplifiers in accordance with this invention, each stage should be carefully screened from the adjacent stage as indicated at 45. The screened valves employed may conveniently be of the transverse electrode type, in which the screening grid 5 is a transverse disc-like electrode effectively dividing the tubular envelope 4 into two parts, one contain- 'ing the input electrodes 3 and 2 and the other 46 in the screening partition 45a between two stages, so that the screening grid 5 lies substantially in the plane of the said partition 45a, which therefore acts as a capacitative continuation of the said screening grid 5. e I

In order that the invention may be more readily carried out, the detailsof one of the coupling transformers which has been successfully used, for example, as transformer 7" of Figure 4, will now be given. It is, of course, to be understood that this design is given only by way of example and guidance.

The core was composed of pairs of laminations of the form shown in Figure 9, butted together and interleaved, i. e. lapped so that there is effectively no air gap in the magnetic circuit. The overall dimensions of each pair of lamina- The arrangement.

tions (when assembled). were 9 cms. by 5.3 cms., the width of the middle limb being 2.4 cms. and thewin'dows being each 2.2 cms. by 3.1 cms. 54 pairs of laminations. of thickness .35 mm. were employed, the insulation between successivepairs being .05 mm.

. The following are the essential physical characteristics of a transformer which has been employed in carrying out the invention:

Inductanceof primary= 0.6 henry Inductance of secondary=22 henries} SOOON' Primary turns=830No. 4-2 S.W.G. enamel covered Wire (one section). Secondary turns=5000 No. 44 S.W.G. enamel covered wire (one section). Sections spaced 4 mm. apart.

Effective self capacityof primary=1500 (J41. F (when used in valve circuit).

. Effective self capacity of secondary, including effect of subsequent valve=42 fL/L F (15 ,Ltpt due to said valve.)

Resonance frequency in valve circuit=5000-.

measurement from a six stage amplifier:--

' Frequency in Voltage amplificycles per second fication It will be noted that these figures are given in terms of voltage amplifications as distinct from the more usually employed figures of power amplification, and it will be seen; therefore, that the magnitude of the amplification at the higher frequencies is very much greater than the amplification normally obtainable from known amplifiers. Indeed, with the amplifier shown in Figure 10, which shows the actual arrangement of a six stage amplifier, successful speech has been obtained in the laboratory over 120 equivalent miles of standard cable. In Figure 10 the chain lines represent electrostatic and electromagnetic screens, while the-numbers over the resonant transformers represent their respective resonance frequencies.

Although triodes have been shown in the last two stages, it is preferable to use pentodes, since these valves have much smaller anode to grid capacity,.whilst greater output is obtainable.

. Where a triode follows a resonance transformer, it is strongly desirable to reduce the input impedance effect on the transformer to a minimum. If the triode is used as a resistance amplifier, its input impedance is mainly that of a capacity connected across the grid and filament. This has been found to lower the resonance frequency of the preceding transformer, thereby reducing the upper speech frequencies. Either a transformer with a secondary having a high value of resonance frequency (apart from the effect of the following valve) should therefore be cmplowed, or both primary and secondary may have (separately) resonance frequencies outside the normal range (lying, say, between 10000 and 15000cycles) a tertiary winding having a suitable resonance frequency in the working range being. coupled electromagnetically to the primary and secondary. A condenser should be connected across the tertiary winding and the electrostatic coupling between the windings should be as small as possible. This arrangement has been found satisfactory and prevents the input impedance of the following valve from substantiallyaffecting the speech characteristics of the amplifier.

Where screened grid valves are employed in the output stages (say the last two stages) of the amplifier, there is less need for precautions, since the input impedance is much higher, assuming that the valves are not utilized for very high magnification, i. e. the whole amplification factor of the valve is not used. Where large magnification is employed, care must, of course, be taken regarding input impedance.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

1. An audio frequency amplification system comprising a multiplicity of screen grid valves each having input and output circuits and coupling means comprising a series path containing resistance and reactance elements directly connecting the output circuit of one screen grid valve with the input circuit of a succeeding screen grid valve, means for short circuiting the reactance element in said coupling means and a capacity element connected in shunt with said reactance element.

2. In an amplification system, a multiplicity of electron tubes each including a cathode, a control grid and an anode and each having input and output circuits interconnecting said electrodes, a resistance element and a choke coil connected in series, a condenser of high impedance to low frequencies and low impedance to high frequencies shunting said choke coil, a resistance connected across the input circuit of the succeeding electron tube andcapacity means interconnecting the output circuit of said first mentioned electron tube with the input circuit of the succeeding" electron tube.

- 3. An audio frequency amplification system comprising a plurality of screen grid valves, each having input and output circuits, a source of potential included in the output circuit of one of said valves, a tap extending from said source of potentialto the screen grid element in the said screen grid valve, a series circuitextending be tween the anode element of the said screen grid valve and the said source of potential, a resistance element and a reactance element included in the said series circuit, a connection intermediate said reactance element and said source of potential to one side of the input circuit of the succeeding screen grid valve, an adjustable tap on said resistance element to the opposite side of the input circuit of the succeeding screen grid valve, a condenser disposed in series with the said tap, and a condenser connected in shunt with said reactance element, said resistance and reactance elements and said condensers being operative overthe audio frequency range of said amplification system.

4. In an amplifying system, the combination of an amplifying valve of high internal impedance and high magnification characteristics, said valve including a cathode and anode, a control grid and a screen grid, an input system connected between said control grid and said cathode, an output circuit connected between said anode and said cathode, a source of potential in said output circuit, a connection between a point on said source of potential and said screen grid, and means for coupling the output circuit of said amplifying valve with the input of a succeeding amplifying'valve comprising a resonant coupling device whose impedance is low as com- 1 pared with the internal impedance of the valve preceding it and which is tuned to a frequency at or near the upper limit of the range of frequencies to which said circuits are responsive.

WILLOUGHBY STATHAM SMITH. NORMAN WILLIAM MCLACHLAN. 

